1. Field of the Invention
The present invention relates to a method for producing polyurethane powder enamels free of blocking agents which have excellent storage stability and which cure above 120.degree. C. into elastic, impact resistant and weather resistant enamel films, as well as the polyurethane powder enamels produced by the process.
2. Description of the Prior Art
In German OS No. 24 20 475 polyurethane powder enamels are described that are produced by the reaction of polyisocyanates having uretidione groups with polyols. However, the examples are limited to the uretidione dimer of toluene diisocyanate and the uretidione dimer of hexamethylene diisocyanate. Also it is necessary in this case to compensate for the higher functional portion of the cross-linking agent with monofunctional reagents, i.e., the uretidione/isocyanurate mixtures must first be linearized.
From German OS No. 25 02 934 it is known to produce PUR-powder enamels, in which polyisocyanates that contain at the same time, latent isocyanate groups in the form of uretidione groups and terminal isocyanate groups blocked with blocking agents, are cured with polyols. Depending on the particular blocking agent used, more or less extensive environmental pollution occurs during the curing process. Furthermore, in this case, the functional groups of the cross-linking component play no part in the production of the enamel.
With the dimerization catalysts belonging to the prior art (tertiary phosphines) it has hitherto not been possible to produce an isocyanurate-free uretidione dimer of isophorone diisocyanate (IPDI). A uretidione dimer of isophorone diisocyanate produced in accordance with German OS No. 16 70 720 or German OS No. 19 34 763 contains an additional ca. 20-40% by weight of IPDI trimer (isocyanurate trimer of isophorone diisocyanate), depending on the reaction conditions.
Thus, it is not surprising that until now no polyurethane powder enamels free of blocking agents were known that were prepared from polyols and linear isophorone diisocyanate polyaddition compounds containing terminal non-blocked NCO groups and uretidione groups, because the prerequisite for producing the above-mentioned polyaddition compounds containing uretidione groups is that the uretidinedione-containing diisocyanate added to extend the chain by reacting with the diols must have an NCO funtionality of 2. When the NCO functionality of the uretidione-containing diisocyanate is &gt;2 at least some gelling must be expected when it is reacted with diols.
The known uretidione dimer of isophorone diisocyanate, which contains isocyanurate, cannot be considered when further reaction with diols is intended in order to prepare useful starting materials for polyurethane chemistry (binding components for PUR powders free of blocking agents).
Production of blocking agent-free polyurethane powder enamels according to the invention was first made possible by the discovery of a catalyst for dimerization of IPDI that permitted the production of a practically isocyanurate-free uretidione dimer of isophorone diisocyanate which can be more than 98% decomposed by heating back into IPDI.
The production of this uretidione-containing isophorone diisocyanate is not the subject of this patent application. It is prepared by dimerizing isophorone diisocyanate, optionally in an inert organic solvent, with the help of a catalyst of the general formula EQU X.sub.m P(NR.sub.2).sub.3-m
wherein
m=0, 1 or 2 PA1 X=Cl, OR or R, PA1 R=identical or different alkyl radicals, aralkyl radicals or cycloalkyl radicals (optionally alkyl substituted), or two radicals together form a ring, PA1 at temperatures of 0.degree.-80.degree. C., preferably 10.degree.-30.degree. C. After a conversion of 5-70%, preferably 20-50%, the 1,3-diazacyclobutanedione-2,4 may be isolated from the reaction mixture, without prior deactivation of the catalyst, as the residue remaining after thin film distillation. Monomer and catalyst may be isolated in the distillate.
The formation of the polyadduct, which is also not claimed in this application, is carried out by reaction of this pure uretidione-containing dimer of isophorone diisocyanate with diols.
Furthermore, production of the polyurethane powder enamels free of blocking agents according to the invention only became possible with the successful preparation of polyhydroxy compounds which, due to their physical and chemical properties, prevent gelling and, due to their low melt viscosity, assure good flow of the enamel films.
It has now been found that extremely high quality polyurethane powder enamels are obtainable by using as cross-linking agents certain isocyanurate-free isophorone diisocyanate polyaddition products which contain uretidione groups, and which are essentially free from isocyanate blocking agents that would split off and become volatile under baking conditions, in combination with polyhydroxy compounds having low melt viscosity and limited hydroxyl functionality.
In addition, the coating media according to the invention have the advantage of providing enameling even in thick layers without film flaws such as bubbles and pits. The fact that no blocking agent is liberated during the curing process constitutes a very substantial advantage. The physiological iritation of workers is avoided, devices to remove offensive odors can be eliminated and there are practically no environmental problems. Furthermore, the economic aspect cannot be ignored, particularly inasmuch as all components are completely incorporated into the enamel film.